This invention relates to a contact-type waste gas combustion device for burning waste gases containing combustible and noxious matters generated in various processes such as photogravure printing, coating, laminating, enamel coating on electric wires, general painting and the like by increasing the temperature of such gases through a heat exchange with regenerative means to a temperature above the ignition temperature of such gases and then surface-burning the gases over and/or within the regenerative means by the use of heat derived from the regenerative menas or contact means as the combustion meduim.
Thinner-vapor from a painting shop, printing ink solvent vapor from a printing shop, solvent-vapor from a laundry shop and waste gases from a petroleum chemical shop are, for example, generally discharged into the atmosphere while the gases still contain unburnt or incompletely burnt combustible materials which tend to pollute the environment if discharged into the atmosphere. However, since these combustible matters are generally contained in a charge of waste gases in very small amounts below several thousands ppm, for example, even if one tried to burn waste gases containing such combustible matters in low concentrations, they will not ignite or flames will not spread in the gases. Thus, the so-called catalyst method, in which waste gases are burnt on the surface of a catalyst or an active substance, has been considered an excellent method.
Although the catalyst method has the advantage that the combustion of waste gases can be carried out at a relatively low temperature, such a method requires the use of an expensive catalyst as well as a relatively large installation. In order to carry out the waste gas combustion continuously, it is generally necessary to burn a combustion support fuel throughout the combustion operation. Furthermore, even if efforts are exerted to maintain the thickness of the catalyst layer uniform throughout the combustion operation, since the catalyst generally is relatively thin, the catalyst layer thickness becomes non-uniform resulting in irregular flow of gases through the catalyst. As a result, a local overheating (hot spots) occurs in the catalyst layer to thereby accelerate the deterioration of the expensive catalyst layer and the catalyst has to be prematurely replaced by a new catalyst layer
In order to prevent occurrence of any local overheating on the catalyst layer, if the thickness of the catalyst layer is increased to an allowable upper limit, the catalyst method still has the disadvantage that gases containing unburnt or partially burnt portions may be discharge into the atmosphere. Combustion conditions for waste gases having varying combustible matter concentrations also vary sensitively because the employed catalyst in the catalyst method has a high activity. Furthermore, the waste gases from the above-mentioned various industrial fields are becoming more complicated year after year and the quantity of combustible matters contained in such industrial waste gases also varies within a wide range. Such being the situation, from the view point of operation conditions, the catalyst method also has the disadvantage that the available temperature range is relatively limited and narrow for the combustion device employed in the catalyst method.
Another of the prior art waste gas combustion methods is the so-called direct combustion method. The direct combustion method has some advantages as compared with the catalyst method in that the direct combustion method requires a relatively smaller installation than that required in catalyst method particularly because the method requires no devices or means necessary in addition with the employment of a catalyst. In the direct combustion method, in order to directly burn waste gases with flames, fuel has to be continuously supplied to maintain the flames at a high temperature to sustain the combustion of the gases, this resulting in increase of fuel cost. Also, in the direct combustion method, a minor portion of waste gases pass through the combustion system in an unheated state and as the result, unburnt material and/or partial oxides are inevitably discharged out into the atmosphere from the system. The thus discharge oxides contain formaline and the like, for example and emit an odor much less desirable and more objectionable than the charge of waste gases or the gases prior to the combustion. In a strict sense, the generation of such unburnt material and/or partial oxides presents a grave problem. This problem is equally common to the catalyst method though the seriousness of the problem may be different from that in connection with the direct combustion method.
Considered from the structural aspect, according to the catalyst method, a charge of waste gases generally flows from a heat exchanger to and into a heating zone where the temperature of the gases is increased to a predetermined value and then burnt over the catalyst layer; the increased temperature gases finally flow back to the heat exchanger to pass through the exchanger from where the gases are discharged out of the system. On the other hand, according to the direct combustion method, a charge of waste gases flows from the heat exchanger directly into a combustion zone where the gases contact flames from a gas burner or the like to be burnt thereby and then flow back through the heat exchanger from where the gases are discharged out of the system.
In a reactor which has a heat exchanger and in which a reaction temperature is maintained with the reaction heat from the reactor itself, the temperature of a catalyst or more particularly, the reaction temperature at the inlet of the catalyst is generally maintained by directly forcing a charge of waste gases at a cold temperature to flow into the inlet of the catalyst prior to the flowing of the gases into the heat exchanger.
When this method is carried out in a heat exchanger having a group of regenerative units, the average temperature of waste gases to be discharged out of the system in the discharge stroke is high enough to deteriously affect the rotary switching valve.